Field
The present disclosure generally relates to a technique for determining a sensible heat flux and/or performing evapotranspiration measurements for use in agriculture. More specifically, the present disclosure relates to a technique for determining a sensible heat flux using a surface-renewal technique without calibration.
Related Art
Evapotranspiration measurements offer the prospect of a significant advance in agriculture. In particular, if the amount of water vaporized from a surface (such as the ground or vegetation covering the ground) can be accurately determined, farmers can optimize the amount of water applied to their crops. This capability would allow farmers to increase crop yield, reduce costs and preserve scarce water resources.
In practice, it is difficult to accurately and economically determine the amount of water vaporized from a surface. For example, one approach to determining the amount of water vaporized from a surface is based on energy conservation. In particular, energy balance dictates that residual heat flux that vaporizes water is given by the net electromagnetic radiation incident on the surface minus the sum of the heat flux conveyed from the surface via air and the heat flux conducted into surface (i.e., the ground). By measuring these parameters, the residual heat flux that vaporizes water can be calculated (and, thus, the amount of water vaporized from the surface can be determined).
However, the accuracy of measurements of the heat flux conveyed from the surface via the air (which is sometimes referred to as the ‘sensible heat flux’), regardless of measurement technique (e.g., eddy covariance, surface renewal, etc.), is often degraded by the size of the temperature sensors that perform these measurements. For example, a temperature at a distance above the surface may be measured using a thermocouple (and, more generally, a temperature sensor). While a very small thermocouple can provide accurate measurements, such a small temperature sensor is often fragile and, therefore, may be unreliable. To address this problem, a larger thermocouple may be used, but the thermal inertia of a larger temperature sensor can distort the temperature measurements. This distortion typically degrades the accuracy of the temperature measurements and, thus, reduces the accuracy of the determined amount of water vaporized from the surface.
Consequently, existing approaches for measuring the heat flux conveyed from the surface (and, thus, for determining the amount of water vaporized from the surface), such as eddy covariance or surface renewal, involve a calibration procedure to compensate for the thermal inertia of the sensor. During this calibration procedure, the temperature measurements can be corrected to give a more accurate estimate of the heat flux conveyed from the surface. In addition, the surface-renewal technique for measuring the heat flux usually requires calibration against other measurement techniques (such as eddy covariance). This calibration is different from the thermal inertia calibration described previously, and it is typically difficult, time-consuming and expensive. Moreover, this calibration procedure for surface-renewal measurements usually has to be performed whenever the heat flux conveyed from the surface is measured.
Hence, what is needed is a technique for determining a sensible heat flux using the surface-renewal technique and/or performing evapotranspiration measurements without the problems described above.